In the past, a bulk-type solar battery cell is generally manufactured by a method explained below. First, for example, a p-type silicon substrate is prepared as a substrate of a first conduction type. A damage layer on a silicon surface that occurs when the silicon substrate is sliced from a casting ingot is removed by thickness of 10 micrometers to 20 micrometers using, for example, several wt % to 20 wt % of caustic soda or carbonate caustic soda. Thereafter, anisotropic etching is performed using a solution obtained by adding IPA (isopropyl alcohol) to the same alkali low concentration solution to form texture such that a silicon (111) surface is exposed. The formation of the texture does not always have to be performed by wet treatment. The texture can be formed by, for example, dry etching (see, for example, Patent Literature 1).
Subsequently, as diffusion treatment, the p-type silicon substrate is treated under, for example, mixed gas atmosphere of phosphorus oxychloride (POCl3), nitrogen, and oxygen at, for example, 800° C. to 900° C. for several ten minutes to uniformly form an n-type layer on the front surface of the surface of the p-type silicon substrate as a dopant layer of a second conduction type. Sheet resistance of the n-type layer uniformly formed on the silicon surface is set to about 30 to 80Ω/square, whereby satisfactory electric characteristics of a solar battery are obtained. Thereafter, the substrate is immersed in a hydrofluoric acid water solution to etch and remove a vitreous material (PSG) deposited on the front surface during the diffusion treatment.
Subsequently, the n-type layer formed in an unnecessary region such as the rear surface of the substrate is removed. The removal of the n-type layer is performed by, after depositing polymeric resist paste on a light-receiving surface side of the substrate and drying the polymeric resist paste by a screen printing method to protect the n-layer formed on the light-receiving surface side of the substrate, immersing the substrate in, for example, 20 wt % of a potassium hydroxide solution for several minutes. Thereafter, the resist is removed using an organic solvent. As another method of removing the n-type layer on the rear surface or the like of the substrate, there is also a method of performing end face separation using laser or dry etching at the end of the process.
Subsequently, an insulating film such as a silicon oxide film, a silicon nitride film, or a titanium oxide film is formed on the front surface of the n-type layer at uniform thickness as an insulating film for preventing reflection (a reflection preventing film). When the silicon nitride film is formed as the reflection preventing film, a film is formed under a condition of reduced pressure and temperature equal to or higher than 300° C. by, for example, a plasma CVD method using a silane (SiH4) gas and an ammonium (NH3) gas as raw materials. The refractive index of the reflection preventing film is about 2.0 to 2.2. Optimum thickness of the reflection preventing film is about 70 nanometers to 90 nanometers. It should be noted that the reflection preventing film formed in this way is an insulator. A front surface side electrode simply formed on the reflection preventing film does not act as a solar battery.
Subsequently, silver paste to be formed as a front surface side electrode is applied on the reflection preventing film in the shapes of a grid electrode and a bus electrode by the screen printing method using masks for grid electrode formation and for bus electrode formation and dried.
Subsequently, rear aluminum electrode paste to be formed as a rear aluminum electrode and rear silver paste to be formed as a rear silver bus electrode are applied on the rear surface of the substrate respectively in the shape of the rear aluminum electrode and the shape of the rear silver electrode by the screen printing method and dried.
Subsequently, the electrode pastes applied on the front and rear surfaces of the silicon substrate are simultaneously baked at about 600° C. to 900° C. for several minutes. Consequently, the grid electrode and the bus electrode are formed on the reflection preventing film as front surface side electrodes and the rear aluminum electrode and the rear silver bus electrode are formed on the rear surface of the silicon substrate as rear surface side electrodes. On the front surface side of the silicon substrate, a silver material comes into contact with silicon and re-solidifies while the reflection preventing film is melted by a glass material included in the silver paste. Consequently, conduction between the surface side electrodes and the silicon substrate (the n-type layer) is secured. Such a process is called fire-through method. The rear aluminum electrode paste also reacts with the rear surface of the silicon substrate. A p+ layer is formed right under the rear aluminum electrode.
To improve the efficiency of the bulk-type solar battery cell formed as explained above, optimization of an unevenness shape of the surface on the light-receiving surface of the substrate, i.e., the shape of texture is important. In the past, concerning the unevenness shape, a shape optimum for one parameter is applied to the entire surface of a cell. For example, Patent Literature 1 discloses that, when the unevenness shape is formed by dry etching, the unevenness shape is optimized using the reflectance of incident light made incident on a solar battery as a parameter. This is because short-circuit current density, which is one of electric characteristics of the solar battery, is improved by selecting a condition under which the reflectance of the incident light is lower. Concerning a method of forming the texture, for example, Patent Literature 2 discloses that dry etching in multiple stages is carried out. This forming method is aimed at obtaining a uniform unevenness shape over the entire surface of a cell.